There has been a great demand in the market for portable communication and computing devices such as a portable TV and cellular phone. All these devices need a small, light-weight and low-power consumption display device, and development efforts have been made accordingly.
FIG. 7 shows a circuit diagram corresponding to a single pixel element of a conventional liquid crystal display device. A gate signal line 51 and a drain signal line 61 are placed on an insulating substrate (not shown in the figure) perpendicular to each other. A pixel element selection TFT 65 connected to the two signal lines 51, 61 is formed near the crossing of the two signal lines 51, 61. The source 11s of the TFT 65 is connected to a pixel electrode 80 of the liquid crystal 21.
A storage capacitor element 85 holds the voltage of the pixel electrode 80 during one field period. A terminal 86, which is one of the terminals of the storage capacitor element 85, is connected to the source 11s of the TFT 65, and the other terminal 87 is provided with a voltage common among all the pixel elements. When a scanning signal is applied to the gate signal line 51, the TFT 65 turns to an on-state.
Accordingly, an analog image signal from the drain signal line 61 is applied to the pixel electrode 80, and the liquid crystal 21 through the pixel electrode 80, and the storage capacitor element 85 holds the voltage. The voltage of the image signal is applied to the liquid crystal 21 through the pixel electrode 80, and the liquid crystal 21 aligns in response to the applied voltage for providing a liquid crystal display image. This configuration is capable of showing both moving images and still images. There is a need for the display to show both a moving image and a still image within a single display. One such example is to show a still image of a battery within area in a moving image of a portable telephone display to show the remaining amount of the battery power.
However, the configuration shown in FIG. 7 requires a continuous rewriting of each display to provide a still image. This is basically to show a still-like image in a moving image mode, and the scanning signal needs to activate the TFT 65 at each scanning.
Accordingly, it is necessary to operate a driver circuit which generates a drive signal for the scanning signals and the image signals, and an external LSI which generates various signals for controlling the timing of the drive circuit, resulting in a consumption of a significant amount of electric power. This is a considerable drawback when such a configuration is used in a portable telephone device, which has only a limited power source. That is, the time a user can use the telephone under one battery charge is considerably short.
Japanese Laid-Open Patent Publication No. Hei 8-194205 discloses another configuration for display device suited for portable applications. This display device has a static memory for each of the pixel elements, as shown in FIG. 8. A static memory, in which two inverters INV1 and INV2 are positively fed back to each other, holds the image signal for reducing the power consumption. In this configuration, a switching element 24 controls the resistance between a reference line and a pixel electrode 80 in response to the divalent digital image signal held by the static memory in order to adjust the biasing of the liquid crystal 21. The common electrode, on the other hand, receives an AC signal Vcom. Ideally, this configuration does not need refreshing the memory when the image stays still for a period of time.
As described above, the display device equipped with a static memory for holding digital image signals is suitable for displaying a still image with shallow depth and reducing the consumption of the electric power.
However, the display device with above configuration has the following problem, which will be explained by referring to FIG. 9. Suppose the source 11s of the pixel element selection TFT 65 is held at L (low) level and the output node of the inverter INV1 is held at H (high) level. Under this circumstance, when a H signal is outputted from the external circuit, fed to the drain signal line 61 and written to the static memory, the electric current goes through from the drain signal line 61 to the TFT 65 to N-channel type TFT as shown by the broken line in the figure, since the N-channel type TFT of the inverter INV2 is in an on-state. Therefore, the H level is effected by the L level, resulting in the possibility of the erroneous writing due to the decrease in H level.
In order to write the H data correctly, the voltage of source 11s of the TFT 65 should be higher than the threshold voltage of the inverter INV1. However, there is a possibility of the decrease in the voltage of the source 11s of the TFT 65 because of the passage of the electric current stated above. Therefore, the following measures can be taken for solving the above problem.    1) To increase the voltage of the H level supplied to the drain signal line 61 from the external circuit.    2) In order to decrease the on-state resistance of the pixel element TFT 65, the voltage should be increased when the gate signal line 51 is selected, or the channel width of the TFT 65 should be expanded.
However, the first measures have the problem of the increased energy consumption due to the increased voltage of the external circuit. And the second measures have problems of the increased voltage of the gate driver and the increased size of the TFT. It also has the difficulty in the layout of the pixel elements with fine pitch.